A resistive bolometric detector usually measures the power of an incident radiation in the infrared range. For this purpose, it comprises an absorbing resistive bolometric element which converts the light flow into a heat flow, which causes a temperature rise of said element with respect to a reference temperature. Such a temperature increase then induces a variation of the electric resistance of the absorbing element, causing voltage or current variations thereacross. Such electric variations form the signal delivered by the sensor.
However, the temperature of the bolometric element usually largely depends on the environment thereof, and especially on the temperature of the substrate which comprises the electronic read circuit. To make the absorbing element as little sensitive as possible to its environment, and thus to increase the detector sensitivity, the bolometric element is generally thermally insulated from the substrate.
FIG. 1 is a simplified perspective view of an elementary state-of-the-art resistive bolometric detector 10 for infrared detection illustrating this thermal insulation principle. Such an elementary detector, usually called “bolometer”, or “bolometric microbridge”, here in the form of a suspended membrane, is conventionally part of a one- or two-dimensional array of elementary detectors.
Bolometer 10 comprises a thin membrane 12 absorbing the incident radiation, suspended above a substrate—support 14 via two conductive anchoring nails 16 to which it is attached by two thermal insulation arms 18. Membrane 12 usually comprises a layer of electric insulator, such as for example SiO2, SiO, SiN, SiON, ZnS or other, which provides the mechanical stiffness of membrane 12, as well as a metallic electric interconnection layer deposited on the insulating layer.
A thin layer 20 of resistive thermometric material is further deposited at the center of membrane 12 on the metal interconnection layer, especially a layer of semiconductor material, such as p- or n-type polysilicon or amorphous silicon, lightly or strongly resistive, or a vanadium oxide (V2O5, VO2) processed in a semiconductor phase, or titanium, titanium oxide (TiOx), or nickel oxide (NiOx). Finally, substrate-support 14 comprises an electronic circuit integrated on a silicon wafer, usually known as “read circuit”. The read circuit comprises, on the one hand, the elements for stimulating and reading from thermometric element 20 and, on the other hand, the multiplexing components which enable to serialize the signals originating from the different thermometric elements present in the array detector.
In operation, membrane 12 heats up under the effect of an incident electromagnetic radiation and the generated heat power is transmitted to layer 20 of thermometric material. Periodically, the read circuit arranged in substrate 14 biases membrane 12 by submitting nails 16 to a bias voltage and collects the current flowing through thermometric element 20 to deduce a variation of the resistance thereof, and thus the incident radiation having caused said variation.
While the fact of suspending the thermometric element above the substrate enables it to undergo an electric resistance variation under the effect of the incident radiation, such a variation however remains minute. Indeed, in the context of the elementary bolometric detector of FIG. 1, at 300° K, a variation by 1 K of the observed scene induces a relative variation of the electric resistance of thermometric layer 20 by approximately 0.04%. Indeed, most of the value of the electric resistance of this element is mainly dictated by the direct environment of the membrane. Particularly, the substrate influences the temperature of membrane 12 via the thermal conduction through nails 16 and arms 18, which sets approximately 70% of the value of the electric resistance of layer 20. In the best case, less than 10% of the value of the electric resistance, and more generally less than 1% thereof, are set by the incident radiation. The most part of the electric resistance of thermometric material layer 20 being set by elements unrelated to the observed scene, when no specific measures are taken, the detector read dynamic range is thus very limited, which makes such a detector very difficult to use.
To overcome this issue, the elementary bolometric detector is associated with a compensation or skimming structure, aiming at removing the non-useful part of the signal originating from the reading from the thermometric element of the membrane, usually known as “common-mode” signal.
FIG. 2 is an electric diagram of an infrared bolometric detector 200 of the state of the art comprising such a skimming structure. Detector 200 comprises a two-dimensional array 202 of unit detection elements 204, or “pixels”, each comprising a sensitive bolometer 206 in the form of a membrane suspended above a substrate, for example, the bolometer illustrated in FIG. 1, connected at one of its terminals to a constant voltage “VDET” and at the other terminal to a MOS biasing transistor 208 setting the voltage across bolometer 206 by means of a gate control voltage “GDET”. Pixel 204 also comprises a selection switch 210, connected between MOS transistor 208 and a node “A” provided for each column of array 202, and driven by a control signal “SELECT”, enabling to select bolometer 206 for the reading therefrom. The two-dimensional assembly of suspended membranes, usually called “retina”, is placed in a tight package in line with a window transparent to the infrared radiation to be detected and in the focal plane of an optical system (not shown). Transistor 208 and switch 210 are usually formed in the substrate under control of the membrane of bolometer 206.
Detector 200 also comprises, at the foot of each column of array 202, a skimming structure 212 comprising a compensation bolometer 214 identical to bolometers 206 of pixels 204 from an electrothermal viewpoint and made insensitive to the incident radiation originating from the scene to be observed by being “thermalized” to substrate 14. The electric resistance of bolometer 214 is thus essentially dictated by the temperature of substrate 14.
Bolometer 214 is further connected at one of its terminals to a constant voltage “VSKIM”, and skimming structure 212 further comprises a biasing MOS transistor 218 setting the voltage across bolometer 214 by means of a gate control voltage “GSKIM” and connected between the other terminal of bolometer 214 and node “A”.
Detector 200 also comprises, at the foot of each column of array 202, an integrator 220 of CTIA type (“capacitive transimpedance amplifier”) for example comprising an amplifier 222 and a capacitance 224 connected between the inverting input and the output of amplifier 222. The inverting terminal and the non-inverting terminal thereof are further respectively connected to node “A” and to a constant voltage “VBUS”. A switch 226, driven by a signal “RAZ”, is also provided in parallel with capacitance 224, for the discharge thereof. The outputs of CTIAs 220 are eventually, for example, connected to respective sample-and-hold devices 228 for the delivery of voltages “Vout” of the CTIAs in multiplexed mode.
Finally, detector 200 comprises a management unit 230 controlling the different previously-described switches. In operation, array 202 is read from line by line. To read from a row of array 202, switches 210 of the line of pixels 204 are turned on and switches 210 of the other lines are turned off. After a phase of discharge of the CTIA capacitors at the foot of the columns, achieved by the turning-on of switches 226 followed by their turning-off, a circuit such as shown in FIG. 3 is thus obtained for each pixel of the line being read. A current “Iop” flows in bolometer 206 of the pixel under the effect of its voltage biasing by MOS transistor 208, and a current “Is” flows in bolometer 214 of the skimming structure under the effect of its voltage biasing by MOS transistor 218. The resulting current difference is integrated by CTIA 220 for a predetermined integration period “Tint”. Output voltage “Vout” of CTIA 220 thus is a measurement of the variation of the resistance of bolometer 206 caused by the incident radiation to be detected since the non-useful part of current “Iop”, that is, the substrate temperature, and thus the ambient temperature, is at least partly compensated for by current “Is” specifically generated to reproduce this non-useful part. Since the substrate temperature is compensated for, it may freely vary and it is thus not needed to provide devices for controlling the substrate temperature, particularly a Peltier cooler.
According to a first specific design of the state of the art, compensation bolometer 214 comprises a stack identical to the stack forming the suspended portion of the membrane of bolometers 206, and thus the suspended membrane of bolometers 206 without the thermal insulation system essentially comprising thermal insulation arms 18, directly formed on top and/or inside of substrate 14. This first configuration however requires a design different from that of sensitive bolometers, which complicates the detector manufacturing.
According to a second specific configuration of the state of the art, compensation bolometer 214 comprises all the elements forming sensitive bolometers 206 of array 202. The membrane of bolometer 214 is thermalized to the substrate by elements having a low thermal resistance, for example, metal elements, rigidly attached to the membrane and to the substrate and arranged at the membrane periphery. These elements thus form thermal short-circuits between the membrane and the substrate so that these elements have the same temperature.
FIGS. 4A and 4B respectively are cross-section views of a sensitive bolometer and of a compensation bolometer of the state of the art, for example, of the type described in relation with FIG. 1, the cross-section views corresponding to cross-section plane IV-IV of FIG. 1. As can be observed, compensation bolometer 214 differs from sensitive bolometer 206 in that it comprises two metal elements 300 arranged on either side of bolometer membrane 12 at the periphery thereof. Such a configuration enables to manufacture compensation bolometer 214 substantially identically to the manufacturing of sensitive bolometer 206, except for elements 300. Due to elements 300, the detector manufacturing is thus simpler and less expensive but the manufacturing of the compensation bolometer however remains complex.
Although the skimming structure just described provides satisfactory results, it can however be observed that the thermalization of compensation bolometers 214 to the substrate is insufficient to exactly compensate for the temperature thereof. Further, thermal short-circuits 300 being formed at the periphery of membrane 12 of compensation bolometers 214, the latter require additional technological manufacturing steps and are more bulky than sensitive bolometers 206, which adversely affects the miniaturization and/or the filling factor of compensation bolometers.
It should be noted that this issue arises whatever the type of circuit used to bias and “read” from the array of sensitive bolometers. In the previously-described state of the art, a voltage biasing is considered. The issue also arises in current-biased detectors, the read signal then being a difference between the voltages across sensitive bolometers and the voltages across compensation bolometers.
Similarly, this issue arises whatever the number of compensation bolometers provided in the detector. Particularly, one compensation bolometer may be provided for each sensitive bolometer. Also, a compensation bolometer may be formed by the series connection of unit compensation bolometers of the type just described in order to decrease, for example, the column noise. The same issue thus arises.